High rates of comorbidity between PTSD and antisocial personality disorder (APD) have been documented in both veteran and civilian samples. This comorbidity is of great concern to both researchers and clinicians. For other mental disorders, comorbidity with APD has been associated with treatment complications, a more severe course of illness, and greater impairment for the suffering individual in a wide variety of areas. To date, the nature of the relationship between these disorders remains unclear. This study seeks to conduct secondary data analysis using the resource presented by the Vietnam Era Twin Registry (VETR) to clarify the factors that influence the established high rates of comorbidity found between Antisocial Personality Disorder (APD) and Posttraumatic Stress Disorder (PTSD) in Vietnam veterans. Previously, the VA-supported Survey of Health (1987) and the NIDA-supported Harvard Twin Study of Drug Abuse and Dependence collected demographic, military service, combat exposure, and diagnostic data on 3,234 male-male pairs of monozygotic (MZ) and dizygotic (DZ) twin pairs who range in age from 38-56 (mean age=45). All pairs are comprised of twins who both served on active military duty during the Vietnam era. These data will be used for two main purposes. First, twin methodology will be used to determine the role familial and genetic influences play in the comorbidity of these two disorders. Second, co-twin control methodology will be used to address the variety of methodological limitations which have plagued research in this area.